In automated industrial machines such as machine tools, industrial robots, and the like, forces are applied to workpieces, and the forces act from the outside because of the manner in which these machines operate. In this case, the forces and moments applied to the machines from the outside must be detected, and a control program that corresponds to these forces and moments must be performed. To ensure that a control program that corresponds to these forces and moments is carried out with a high degree of precision, the externally applied forces and moments must be accurately detected.
In view of this situation, various types of force sensors have been proposed in the past. When classified based on the detection scheme, force sensors are usually divided into elastic force sensors and equilibrium force sensors. Elastic force sensors measure a force on the basis of the amount of deformation proportional to the external force. Equilibrium force sensors measure a force by balancing this force with a known force.
Also known are force sensors whose structure is based on the principle that a plurality of strain resistance elements is installed in parts of a strain-generating body which undergoes elastic deformation in accordance with an external force. When an external force is applied to the strain-generating body of the force sensor, electrical signals corresponding to the degree of deformation of the strain-generating body are output from the plurality of strain resistance elements. Forces that have two or more components and are applied to the strain-generating body can be detected on the basis of these electrical signals. The measurement of the force generated by the force sensor can be calculated on the basis of these electrical signals.
Six-axis force sensors constitute a known class of force sensors. Such six-axis force sensors are a class of elastic force sensors that comprise a plurality of strain resistance elements in parts of a strain-generating body. Six-axis sensors divide an external force into force components (forces: Fx, Fy, Fz) in the respective axial directions of three axes (X axis, Y axis, Z axis) of an orthogonal coordinate system, and into moment components (moments: Mx, My, Mz) about the respective axes, and detect the force as six-axis components.
The present inventors previously proposed a six-axis force sensor having a novel construction (see JP 2003-207405 A1). This six-axis force sensor can solve the problem of interference from other axes that prevents the respective components (forces and moments) of an external force applied to a strain-generating body from being resolved with good precision. In this six-axis sensor, a plurality of strain resistance elements is integrally assembled in a specified arrangement pattern on parts of a strain-generating body on a semiconductor substrate by using a semiconductor manufacturing process. The six-axis sensor is constructed from a supporting part which comprises a plate-form semiconductor substrate that has a substantially square plane shape, and which supports an operating part as the peripheral part of the substrate; an operating part having a substantially square plane shape which is positioned in the central part; and connecting parts which connect each of the four sides of the square operating part with the corresponding portions of the supporting part. The strain resistance elements are installed in the boundary parts between the sides of the square operating part and the connecting parts. This six-axis force sensor solves the problem of interference from other axes by improving the configuration of the parts of the strain-generating body and the arrangement pattern of the plurality of strain resistance elements, and optimizing the arrangement pattern of the plurality of strain resistance elements.
JP 2001-264198 A1 may also be cited as a prior art document relating to the invention of the present application. In the multiaxial force sensor disclosed in JP 2001-264198 A1, only some of the connecting parts of the sensor chip are made thinner by electrochemical etching in order to increase the amount of strain of the sensor chip as the sensor chip becomes more compact, and the strain detection sensitivity is increased.
In the six-axis force sensor described in JP 2003-207405 A1, there is no mention of any improvement of the detection balance between the axial forces when the planar shape of the semiconductor substrate that forms the sensor chip is made even more compact. In actuality, the thickness of the semiconductor substrate must also be reduced by the same ratio as the plane surface area in order to make the planar shape of the semiconductor substrate more compact while maintaining a favorable detection balance between the respective axial forces. The reason for this is that even in cases where the external forces applied in the X, Y and Z axial directions are equal, a difference is generated between the amount of deformation of the chip caused by the external forces applied in the X and Y axial directions and the amount of deformation of the chip caused by the external force applied in the Z axial direction if the planar shape alone is made more compact without varying the thickness, and it is therefore impossible to maintain the detection balance. However, there is no description of reducing the thickness of the semiconductor substrate from the standpoint of maintaining a favorable detection balance. Furthermore, the following problem arises in the semiconductor process when an attempt is made to reduce the entire chip to the desired thinness from the start. Since the silicon wafer that is used to manufacture the chip is inherently thin as such in comparison with the surface area of the wafer, a further reduction in the thickness leads to a danger of warping and cracking of the wafer during the process. However, leaving the thickness unchanged relative to the surface area of the chip makes it difficult to increase the chip sensitivity even if flexible parts are formed.
On the other hand, in the multiaxial force sensor described in JP 2001-264198 A1, it is indicated that the thickness of some of the connecting parts of the sensor chip is reduced for the purpose of heightening the strain detection sensitivity of the sensor chip. Here, the rigidity of the connecting parts is lowered to achieve flexibility and make the chip more compact by forming the connecting parts into thin parts using a semiconductor process (electrochemical etching) rather than mechanical working. However, in this multiaxial force sensor, the problem of damage caused by the concentration of stress arises when boundaries of the thin areas (boundaries of thick parts and thin parts) are formed in the connecting parts during reduction of the thickness of the sensor chip. Furthermore, forming boundaries of the thin areas in the connecting parts causes a variation to be generated in the amount of strain detected by the respective strain resistance elements. There is therefore a possibility that a difference will arise between the amount of strain intended in the design and the amount of strain that is actually generated. In other words, an unnecessary concentration of stress tends to occur, and unexpected noise is superimposed on the amount of detected strain as a result of boundaries of the thin areas being formed in the connecting parts. Consequently, individual differences are generated between the respective axes, and hence between the respective chips.